Outboard propulsion system for vessels

ABSTRACT

An outboard propulsion system for vessels includes a supporting structure ( 4 ) to be anchored to the transom wall ( 5 ) of the vessel and a plurality of propellers ( 3 ) carried by the supporting structure ( 4 ). Each propeller ( 3 ) is associated to an electric motor ( 6 ) with a toroid geometry, having an annular rotor (R) rotatable within an annular stator (S) and defining therewithin a central aperture (A), with the blades ( 3 A) of the propeller ( 3 ) which are carried by the rotor (R) and extend to the central aperture (A).

FIELD OF THE INVENTION

The present invention relates to the field of outboard propulsionsystems for vessels, of the type comprising a supporting structure to beanchored to a transom wall of the vessel and at least one propellercarried by said supporting structure.

PRIOR ART

Document U.S. Pat. No. 3,914,629 A discloses an outboard propulsionsystem according to the preamble of claim 1. Marine propellers with anelectric motor having a toroid configuration and radial blades whichextend from the wall of the rotor into the central aperture of the rotorare known from US 2003/0186601 A1, EP 2 332 824 A1, EP 2 591 993 A1, EP2 591 995 A1.

OBJECT OF THE INVENTION

The object of the invention is that of providing a system of this typewhich is characterized by high efficiency, lightness and reduceddimensions.

SUMMARY OF THE INVENTION

In view of achieving this object, the invention provides an outboardpropulsion system for vessels comprising a supporting structure to beanchored to the transom wall of the vessel of one or more propellerscarried by said supporting structure, in which each propeller isassociated with an electric motor having a toroid configuration with anannular rotor rotatable within an annular stator and on its turndefining a central aperture therewithin, and wherein the annular body ofthe stator defines a guiding tube for the flow of water which isaffected by the propeller, characterized in that said one or more bladesare radial blades each extending into said central aperture startingfrom a wall of the rotor towards a blade free end which is located at adistance from the central axis of the propeller, and in that theconfiguration of said one or more blades is such as to cause the waterflow through said central aperture to converge towards a focusing arealocated at the rear of the propeller, with reference to the direction ofmovement of the vessel.

Studies and tests of the Applicant have shown that due to these featuresand in particular due to the converging flow generated by the propeller,the propulsion thrust increases considerably with respect to the knownsolutions.

Preferably, the distance between said focusing area and the median planeof the propeller amounts to 1.8-2.2 times the inner diameter of saidguiding tube at said median plane.

In one embodiment, each propeller comprises a single radial blade.Contrary to what one could think, the propulsion efficiency of apropeller with a single blade which is rotated at high speed is greaterthan that of a propeller with more blades, in which the hydrodynamicfield of one blade is disturbed by that of the other blades. Thepotential drawback of a propeller with a single blade is that it isdynamically unbalanced, but this drawback has no detrimentalconsequences in the case of the invention, since the propeller isintegrated within a motor with a toroid configuration, whose structuresupports the centrifugal forces.

According to a preferred embodiment of the invention, the convergingflow downstream of each propeller is obtained due to that each of saidone or more radial blades has a curved tip oriented towards saidfocusing area and having a configuration such as to generate therequired degree of convergence.

Electric machines adapted to be employed in the propulsion systemaccording to the invention have been developed in recent years for useas motors or generators. A particularly interesting solution for theinvention is that disclosed in European Patent EP 1 885 047 B1 andcorresponding U.S. Pat. No. 7,592,712 B2 of the same present inventor.

In the preferred embodiment, the stator of the motor includes an annularcore of ferromagnetic material having an alternated arrangement of slotsand teeth for receiving the stator windings, and the rotor has anannular core of ferromagnetic material and an annular arrangement ofpermanent magnets arranged radially on the outside of said core, so asto define a plurality of magnetic North-South poles adapted to cooperatewith the stator windings.

In one variant, the toroid electric motor is provided with windings andpermanent magnets arranged only along a portion, such as 70%, of theentire circumferential extension of the toroid motor, so that at theremaining portion of the circumferential extension of the motor, thestator and the rotor can be made with a reduced dimension in thedirection parallel to the motor axis, to advantage of a lowerhydrodynamic drag.

According to a further preferred feature of the invention, the structuresupporting all the propellers, or each propeller, is pivotally mountedaround a horizontal axis transverse with respect to the direction ofmovement and actuating means are provided for adjusting the position ofthis supporting structure around said transverse axis, so as to keep thedirection of the propeller thrust substantially parallel to thehorizontal direction of movement.

Preferably, said actuating means comprise an electric actuator and anelectronic control unit for driving the electrical actuator on the basisof the output signal from a sensor detecting the vessel attitude, suchas an inertial sensor.

According to a further embodiment, which however does not form part ofthe annexed claims, a propulsion unit of the above indicated type can beused also for aerial propulsion in small unmanned aircrafts of theVertical Take-off and Landing (VTOL) type, which can be used for exampleas drones equipped with a camera or a video camera for surveillanceactivities or for conveying small payloads. A vehicle of this type canbe equipped for example with three or more propulsion units withvertical axes arranged around a central supporting structure, which forexample can house an onboard electronic unit and transmitting/receivingmeans for communication with a control station. The configuration ofeach propulsion unit is identical to that described above for theoutboard propeller device, with the difference that in this case theoptimal configuration of the blades of the propellers is that whichcauses the air flow through each rotor to converge into a focusing areaarranged at a distance which is 2.8-3.2 times the inner diameter of therotor. Due to this feature, an aerial propulsion unit according to thisembodiment has a greater efficiency and provides a greater lift withrespect to the conventional solutions in the field of aerial propulsionfor VTOL aircrafts. The known solutions of VTOL aircrafts with three,four or more rotors, both in the version with free rotors and in theversion with ducted rotors, are penalized by an excessive aerodynamicdrag due to the edge effects on the conventional blades and on the bodyof the motor, which is arranged at the centre of each rotor. For a givensurface covered by the rotors, the losses increase with the number ofrotors and with decreasing values of the diameters of the rotors. Thehigher efficiency and the control on the convergence of the flow throughthe annular rotors proposed according to this embodiment enable agreater fuel range of the aircraft. The annular rotor according to thisembodiment is further characterized by a reduction of the aerodynamicnoise, which in conventional rotors is caused by the turbulenceoriginated by the edge effects on the conventional blades.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentfrom the following description with reference to the annexed drawings,given purely by way of non limiting example, in which:

FIG. 1 is a perspective view of one example of a vessel to which anoutboard propulsion system according to the invention is applied,

FIG. 2 shows the vessel of FIG. 1 with a foldable roof structure,carrying an array of solar cells, and illustrated in the deployingstage,

FIG. 3 is a perspective view at an enlarged scale of the propulsionsystem according to the invention,

FIG. 4 is a perspective view at an enlarged scale of the detail of asingle propeller,

FIG. 5 is a perspective view similar to that of FIG. 4, in which thepropellers and the outer housing of the motor driving the propeller havebeen removed, in order to show the supporting frame, with the toroidelectric motor associated therewith,

FIG. 6 is a perspective view of the supporting structure only,

FIGS. 7, 8 are a cross-sectional view and a front view of a singlemotor, with the propeller associated thereto, in a neutral adjustmentposition,

FIG. 9, 10 are views in cross-section similar to that of FIG. 7, whichshow the motor unit with the associated propeller in two end positionsof the movement for adjustment of the unit inclination,

FIG. 11 is a perspective view partially in cross-section of an electricmachine in according to the prior art, originally studied for operatingas generator in association to a rotor driven by an air flow, which isinstead used as a motor for driving the propeller of the systemaccording to the invention,

FIG. 12A show a cross-sectional view disclosing the configuration of thewater flow generated by each propeller of the propulsion systemaccording to the invention, with reference to the example in which thepropeller comprises a single blade,

FIG. 12B-12E respectively show a lateral view, a front view and twoperspective views of the unit of FIG. 12 A,

FIG. 13A shows a further application of the concepts which are at thebasis of the present invention, and

FIG. 13B shows a cross-section taken along line XIII of FIG. 13A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, numeral 1 generally designates an outboard propulsion systemfor a vessel 2. It must be noted that the specific type of vessel shownin the drawings is given here purely by way of non-limiting example,while it is clearly apparent from the following description that thepropulsion system according to the invention is applicable to anunlimited number of different types of vessel, as well as also tosubmarine vessels.

With reference also to FIGS. 2, 3, the propulsion 1 comprises aplurality of propellers 3 (in the specific case which is illustratedherein there are provided three propellers) carried by a supportingframe 4 provided with anchoring means of any known type, for anchoringto the transom wall 5 of the vessel 2.

According to an essential feature of the invention, each propeller 3 isassociated to an electric motor 6 with a toroid geometry having anannular rotor R (see in particular FIG. 4) rotatable inside an annularstator S and defining therewithin a central aperture A into which thereextend the blades 3A of propeller 3. As shown in FIG. 4, the toroid bodyof the electric motor 6 defined by the annular bodies of the rotor andthe stator forms a tube guiding and directing the water flow affected bythe propeller 3. The blades 3A of propeller 3 are radial blades eachextending starting from the wall of the rotor R to a substantiallypointed free end arranged at a distance from axis X of the propeller.

As shown in FIGS. 12A-12E, an important feature of the invention lies inthat the shape of the blades (or of the single blade) of the propelleris such as to cause the water flow through aperture A to convergetowards a focusing area P arranged at the rear of the propeller, withreference to the direction of movement of the vessel. In the case of thesolution shown herein, there is provided a single blade, with theadvantages which have been discussed above. However, a converging flowcan be obtained also with propellers having more than one blade.

Studies and tests of the applicant have shown that the converging flowgenerated by the propeller produces an increase of the propulsion thrustwith respect to the known solutions. The 3D geometry of each blade isselected so as to ensure maximum efficiency and maximum thrust bycausing the flow to converge at a predetermined distance from the planecontaining the rotor.

In the preferred embodiment, each of said one or more radial blades hasa curved tip oriented towards said focusing area, the curvature and the3D shape of the blade have been optimized to obtain the required degreeof convergence.

Preferably, the distance between said focusing area P and the medianplane Z (see FIG. 12) of the blade is 1.8-2.2 times the inner diameterof said guide tube at said median plane. The distance of the focusingarea P from the median plane Z naturally depends from the density of thefluid and in particular for water is preferably 1.8-2.2 times the innerdiameter of the tube. This is indeed the configuration which providesoptimum results in terms of thrust obtained for a given motor power.

As already indicated in the foregoing, electric machines adapted to bemade with the toroid configuration shown in FIG. 4 were already proposedand developed in the past for different applications.

FIG. 11 of the annexed drawings is taken from document EP 1 885 047 B1and the corresponding document U.S. Pat. No. 7,592,712 B2 of the sameinventor. In the case of these documents, the electric machine was usedas an electric generator in association to a rotor rotated by windpower.

In FIG. 11, the body of stator S and the body of rotor R are illustratedin cross-section in order to show the components inside thereof. Firstof all, in the case of the embodiment shown in FIG. 11, rotor R isrotatably supported within stator S by means of magnetic sustenance,obtained through two pairs of annular permanent magnets M_(R) and M_(S).In FIG. 11, by N there is designated an annular core forming part ofstator S, made of ferromagnetic material, such as SMC (“Soft MagneticComposite”). Starting from the inner surface of annular core N there areformed slots C adapted to receive windings (not shown) associated withstator S and adapted to cooperate with permanent magnets carried byrotor R. By D there are designated teeth defined between each other slotC. Rotor R includes an annular core G, which also is made offerromagnetic material, such as SMC. Rotor R further has a annularseries of permanent magnets P arranged radially at the outside of coreG, these magnets being arranged so as to provide an alternatedarrangement of North-South magnetic poles which are to cooperate withthe windings of stator S for generating a rotation of rotor R whenelectric current is passed through the windings. In one variant, therotor can be constituted by a plurality of layers of magnetic compositematerial arranged so as to define an alternated arrangement of magneticNorth-South poles. In another variant, rotor R can be constituted by aplurality of permanent magnets arranged according to a so-called“Halbach array” configuration, known per se, in order to reduce weightand dimensions of rotor R. From the inner surface of the body of rotor Rthere extend radial blades 3A of propeller 3.

As shown in the drawings, in the embodiment illustrated herein theblades 3A of each propeller 3 do not meet at the centre of the rotor,but remain instead spaced from each other, so as to leave a central partof the aperture A within the rotor free. It is however possible toprovide for the blades 3A to join each other at the centre of apertureA.

In one embodiment, the toroid electric motor is made with windings andpermanent magnets arranged only along a portion and, for example 70%, ofthe entire circumferential extension, so that at the remaining portionof the circumferential extension of the motor, the stator and the rotorcan be made with a reduced dimension along the direction parallel to theaxis of the motor, with the advantageous result of a lower hydrodynamicdrag.

Naturally, although the above described electric motor is considered tobe most adapted to be applied to the system according to the invention,it does not represent the only possible solution. From the stand pointof efficiency purely, the electric machine with surface permanentmagnets represents the best choice. However it also involves somedrawbacks:

-   -   the motor with permanent magnets (PM) require special safety        solutions for the power converter; in the case of a failure, the        movement of the PM machine generates an electromotive force        (emf) which must be properly handled in order to avoid further        damages (such as to the battery) and further electric dangers;    -   machines purely of a PM type suffer for the poor availability of        the main raw material, the neodynium-iron-boron for the magnets,        whose production is for the most part in China and subject to        restrictions, due to strategical and environment reasons.

Any other type of electric motor which has a toroid configurationsimilar to that described above can also be used. Induction electricmachines have a lower efficiency but are produced on a large scale andexploit a consolidated and low-cost technology. A better compromise interms of costs, efficiency and safety is represented by reluctancemachines, both of the synchronous type and of the switched reluctancetype.

Compared with conventional solutions, the axial flow machines havefavourable features with respect to efficiency and specific torque. Anaxial-field electric motor comprises a rotatable rotor andgenerators/paths with multiple axial flows (permanent magnets, variablereluctance channels, squirrel-type cage elements) carried by the rotor.The axial flow generators are oriented so that the magnetic flowgenerated thereby is oriented axially at least for a substantialportion. The axial-flow generators are positioned around the rotor withan alternated orientation of the flow direction, so that the directionof flow of adjacent segments is oriented axially at least for asubstantial portion, but in opposite directions. Axial-flow machines canbe made with reduced or zero contents of rare earth elements and forthis reason are of low cost.

As shown in FIG. 5, the body of the stator is mounted within a frame 7forming part of the supporting structure 4.

In one embodiment, the frame 7 which supports each propeller 3 ismounted so that it can be independently oscillate around a horizontaltransverse axis 8 on the supporting structure 4. Alternatively, asolution of the type shown in FIG. 3 may be provided, in which theentire supporting frame 4 is pivotally mounted around a horizontaltransverse axis 9 on the auxiliary structure 10 which is provided withmeans for anchoring to the transom wall 5. In both cases, there areprovided actuating means for adjusting the inclination of the propellersaround a horizontal axis, transverse with respect to the direction ofmovement of the vessel. In the case of the embodiment shown in FIG. 3,this is obtained by an actuating cylinder 11 carried by the auxiliarystructure 10 and driving a crank 12 carried by a shaft 13 carrying thesupporting structure 4 and rotatably mounted around the transverse axis9 on the auxiliary structure 10.

Preferably, actuator 11 is an electric powered actuator, which iscontrolled by an electronic control unit on the basis of output signalsfrom an attitude sensor, preferably of the inertial type, which detectsthe attitude of the vessel and corrects the inclination of thepropellers 3 so as to keep the direction of the thrust parallel to thehorizontal direction of movement of the vessel. Alternatively, thesensor may be an accelerometer or an inclinometer developed preferablyaccording to MEMS (Micro Electro Mechanical Sensor) technology.

The electric motors 6 are supplied by cables (not shown in the drawings)associated to the supporting frame 4 and connected to supply electricbatteries provided within a containing compartments. In the case of theillustrated example, this compartment is provided at the stern of thevessel and is closed by a lid (see FIG. 1).

With reference to FIG. 2, vessel 2 is preferably provided with afoldable panel 15 carrying an array of photovoltaic cells 16, which canbe deployed in a roof fashion above the vessel 2, by means of asupporting frame 17. The energy generated by the photovoltaic cells 16is used to recharge the supply batteries of the electric motors 6.

FIG. 7 shows a propeller 3 in cross-section, according to the variant ofFIGS. 4-6, where the supporting frame of each propeller is independentlyarticulated to the supporting structure 4 around the transverse axis 8.FIGS. 9, 10 show two end positions of the movement for adjusting theinclination of propeller 3 around the transverse axis 8. In theillustrated actual case, the maximum inclination upwardly is 10°,whereas the maximum inclination downwardly is 5°.

According to a further feature, an electronic control unit is providedadapted to control in a differentiated manner at least two motors whichare arranged at the two sides of the median axis of the vessel, tocontrol the direction of movement of the vessel, with no need of a helm.

The propellers are adapted to be rotated by a flow of water so that therespective electric motors can be used as generators for recharging thesupply batteries. These batteries may be recharged also through aninverter AC/DC and supply cable connected to the electric supply (ACrecharge) or by means of a DC-DC converter and other batteries (DCrecharge) according to solutions which are known in the field ofelectric vehicles.

As already indicated above, according to a further embodiment, whichhowever does not form the subject of the annexed claims, a propulsionunit of the above described type can be also provided for aerialpropulsion of small unmanned aircrafts of the VTOL type which can beused for example as drones for surveillance activities, with the aid ofcameras or video cameras, or for conveying small payloads. A vehicle ofthis type can be provided for example with three or more propulsionunits with vertical axes arranged around a central supporting structure,which for example can house an onboard electronic unit andtransmitting/receiving means of communication with a control station.The configuration of each propulsion unit is identical to that describedabove for the outboard propulsion device, with the difference that inthis case the best configuration of the blades of the propeller is thatwhich causes the air flow through each rotor to converge into a focusingarea located at a distance of about 2.8-3.2 times the inner diameter ofthe rotor. Due to this feature, an aerial propulsion unit according tothe invention has a greater efficiency and provides a greater lift withrespect to convention solutions in the field of propulsion units forVTOL aircrafts.

FIG. 13A of the annexed drawings shows a small unmanned aircraft. Purelyby a way of example, the space covered in plan view by this vehicle maybe a square with a side having a length between 5 cm and 50 cm. Acentral supporting structure carries a plurality of (for example four)propeller units 6 arranged therearound, whose structure andconfiguration corresponds for example to that shown in the foregoingdescription. This applies both to the shape of the blade (with thedifference that the focusing distance of the flow is preferably thatindicated above of 2.8-3.2 times the inner diameter of the rotor) and tothe configuration of each electric motor.

This embodiment relates therefore to the following characteristics:

A) A VTOL aircraft, comprising a central supporting structure carrying aplurality of aerial propeller units arranged around it and mainlycharacterized in that:

-   -   each propeller (3) is associated to an electric motor (6) having        a toroid configuration, with an annular rotor (R) rotatable        within an annular stator (S) and on its turn defining        therewithin a central aperture (A), said propeller (3) having        one or more blades (3A) which are carried by the rotor (R) and        extend into said central aperture (A),    -   the annular body of the rotor (R) and the annular body of the        stator (S) define a guide tube for the airflow affected by the        propeller (3),    -   said one or more blades (3A) are radial blades each extending        into said central aperture (A), starting from a wall of the        rotor (R) and ending at a blade free end arranged at a distance        from the central axis of the propeller and,    -   the configuration of said one or more blades (3A) is such as to        cause the airflow through said central aperture (A) to converge        towards a focusing area (P).

B) According to a further preferred feature, the aircraft is furthercharacterized in that the distance between said focusing area and themedian plane of each propeller (3) is 2.8-3.2 times the inner diameterof said guide tube at said median plane.

C) Independently from the value of the focusing distance, according to afurther preferred feature, said aircraft according to feature A) or B)is further characterized in that each propeller (3) comprises a singleradial blade.

D) Independently from the number of blades of each propeller, theaircraft according to any of features A, B or C is further characterizedin that each blade preferably has a curved tip which is oriented towardsthe focusing area (P).

Naturally, while the principle of the invention remains the same, thedetails of construction and the embodiments may widely vary with respectto what has been described and illustrated purely by way of example,without departing from the scope of the present invention.

1. An outboard propulsion system for vessels, comprising a supportingstructure to be anchored to a transom wall of the vessel and one or morepropellers carried by said supporting structure, wherein each propelleris associated to an electric motor with a toroid configuration, havingan annular rotor rotatable within an annular stator and on its turndefining a central aperture therewithin, said propeller having one ormore blades which are carried by the rotor and extending into saidcentral aperture, and wherein the annular body of the rotor and theannular body of the stator define a guide tube for the flow of waterwhich is affected by the propeller, wherein one or more blades areradial blades each extending into said central aperture, starting from awall of the rotor and ending at a blade free end which is located at adistance from the central axis of the propeller, and wherein theconfiguration of said one or more blades is such as to cause the flow ofwater through said central aperture to converge towards a focusing arealocated at the rear of the propeller, with reference to the direction ofmovement of the vessel.
 2. The outboard propulsion system according toclaim 1, wherein the distance between said focusing area and the medianplane of the propeller is 1.8-2.2 times the inner diameter of said guidetube at said median plane.
 3. The outboard propulsion system accordingto claim 1 wherein each propeller comprises a single radial blade. 4.The outboard propulsion system according to claim 1, wherein each ofsaid one or more radial blades has a curved tip oriented towards saidfocusing area.
 5. The outboard propulsion system according to claim 1,wherein the supporting structure of each propeller is pivotally mountedaround a horizontal axis transverse with respect to the direction ofmovement and in that there are provided actuating means for adjustingthe position of the supporting structure around the said transverse axisso as to keep the direction of the thrust of the propellerssubstantially parallel to the horizontal direction of movement of thevessel.
 6. The outboard propulsion system according to claim 5, whereinsaid actuating means comprise an electric actuator and an electroniccontrol unit for driving said electric actuator on the basis of anoutput signal from a sensor detecting the attitude of the vessel, suchas an inertial sensor or an accelerometer or an inclinometer developedpreferably according to the MEMS (Micro Electro Mechanical Sensor)technology.
 7. The outboard propulsion system according to claim 1,wherein there is provided an electronic control unit of the motorsadapted to control at least two motors arranged at the two sides of themedian axis of the vessel in a differentiated manner, for controllingthe direction of movement of the vessel, with no need of a helm.
 8. Theoutboard propulsion system according to claim 1, wherein the statorincludes an annular core of ferromagnetic material having an alternatedarrangement of slots and teeth for receiving the stator windings, and inthat the rotor has an annular core of ferromagnetic material and anannular arrangement of permanent magnets arranged radially at theoutside of said core so as to define a plurality of North-South magneticpoles adapted to cooperate with the windings of the stator.
 9. Theoutboard propulsion system according to claim 8, wherein the toroidelectric motor is provided with an active portion constituted bywindings and permanent magnets arranged only alone a portion, forexample 70%, of the entire circumferential extension of the toroidmotor, so that at the remaining portion of the circumferential extensionof the motor the stator and the rotor can be made with a reduceddimension along a direction parallel to the axis of the motor, toadvantage of a lower hydrodynamic drag.
 10. The outboard propulsionsystem according to claim 1, comprising a foldable panel which can beused in a roof fashion above the vessel and carries an array ofphotovoltaic cells for charging the batteries supplying the electricmotors.
 11. The outboard propulsion system according to claim 1, whereinsaid propellers are adapted to be rotated by a flow of water, so thatthe respective electric motors can be used as generators for rechargingthe supply batteries.
 12. The outboard propulsion system according toclaim 1, wherein the supply batteries of the electric motors can berecharged also by means of an inverter AC/DC and a supply cableconnected to the electric supply (AC recharge) or by a DC/DC converterand other batteries (DC recharge) according to solutions non per se inthe field of electric vehicles.